Two new studies give one more reason to eat a diet rich in fish: prevention of age-related macular degeneration, the leading cause of blindness in old age.
The omega-3 fatty acids found in fish such as salmon are already known to help the heart and brain stay healthy. The new studies, appearing Monday in the Archives of Ophthalmology, add to evidence that fish eaters also protect the eyes.
The new studies aren't the strongest level of scientific evidence, but they confirm the findings of previous studies that also link fish consumption with prevention of macular degeneration.
A study of 681 elderly American men showed that those who ate fish twice a week had a 36 percent lower risk of macular degeneration. In the other study, which followed 2,335 Australian men and women over five years, people who ate fish just once a week reduced their risk by 40 percent.
The U.S. study also found that smokers nearly doubled their risk of the eye condition compared to people who never smoked.
Macular degeneration starts with blurring in the center of what the eye sees. It progresses to blindness, slowly or quickly depending on the type of disease. Six to 8 percent of people age 75 and older have an advanced form of the disease.
"We have a longer life expectancy so the prevalence and burden related to age-related macular degeneration will continue to increase," said Dr. Johanna Seddon of the Massachusetts Eye and Ear Infirmary in Boston, lead author of the U.S. study.
The proper balance of essential fatty acids was crucial to preventing eye disease in the study, Seddon said. The men who ate not only more omega-3 fatty acids, but also fewer omega-6 fatty acids, found in vegetable oils and baked goods, got the most benefit.
Both studies on the effect of fish were based on participants' recall of what they ate. The studies were observational, meaning they observed people's behavior and health. Although the researchers tried to account for other risk factors, the people who ate more fish may have had other healthy habits that lowered their risk.
Stronger evidence may come in five or six years with results from a large, randomized study of how fish oil and another nutrient, lutein, affect macular degeneration, said Dr. Emily Chew of the National Eye Institute, who is heading that study.
Volunteers will be assigned randomly to get either fish oil, lutein, or both - or placebos.
Researchers don't yet know why eating fish seems to protect the eyes. Omega-3 fatty acids may neutralize free radicals in the eye, preventing the formation of new blood vessels, reducing inflammation or all three, Chew said.
Dr. Yu Guang He of the University of Texas Southwestern Medical Center said the new studies confirm findings from other research and will give doctors even more confidence as they advise patients what they can eat to protect their eyesight.